In addition to being important for normal fetal and postnatal growth, there is increasing evidence that insulin-like growth factors I and II (IGF-I, IGF-II) also support the growth of certain cancers. Biologic responses to IGF-I and IGF-II are signaled by the IGF-I receptor. Therefore, we are focusing our research effort on understanding signaling by the IGF-I receptor. We have used the yeast two-hybrid system to identify new binding partners for the IGF-I receptor. We have inserted the cytoplasmic domain of the IGF-I receptor into the LexA DNA binding vector. Poly A mRNA was prepared from the human osteosarcoma cell line, MG-63, and given to a commercial laboratory for preparation of a cDNA library in the yeast two-hybrid activation domain vector. The library screen identified IGF-I receptor binding partners that we and others had previously identified (p85 regulatory subunit of PI3-K, p66 Shc, Grb10, 14-3-3 beta and zeta, SOCS-1, SH2-B/PSM, RACK-1) as well as new interactors (STAT5a, GIPC, Ril, and 4 clones that are not fully characterized). We are focusing on GIPC which represented 25% of the most strongly interacting clones. GIPC did not interact with the insulin receptor in the yeast two hybrid assay. GIPC is a 333 amino acid protein with a central SH2 domain. Mutational analysis in the yeast two-hybrid system showed that GIPC PDZ domain binds to the carboxy tail of the IGF-I receptor. GIPC dimerizes and the dimerization domain has been shown to include the amino terminal domain as well as a portion of the PDZ domain. IGF-I receptor constructs that have been mutated to prevent GIPC binding have been transfected into IGF-I receptor null mouse embryo fibroblasts in order to explore the function of GIPC in IGF-I receptor signaling. We have developed a mouse monoclonal antibody (4G11) directed against the human IGF-I receptor. This monoclonal antibody blocks the binding of radiolabeled IGF-I to MCF-7 breast cancer cells and MG-63 osteosarcoma cells and downregulates the receptor.